Re: [Qemu-devel] [PATCH] xen_disk: convert discard input to byte ranges

[Eric Blake <eblake@redhat.com>]

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On 11/22/2016 10:12 AM, Olaf Hering wrote:
> On Fri, Nov 18, Eric Blake wrote:
> 
>> if (sec_start > (INT64_MAX >> BDRV_SECTOR_BITS) - sec_count)
> 
> I have looked at this for a while now and cant spot how this would cover
> all cases. Are you saying there should be just a single overflow check,
> yours? My change has two: one to check for wrap around and to check
> against the upper limit. My check happens to work with 0/UINT64_MAX or
> INT64_MAX/INT64_MAX as input, yours appearently not.
> Obviously I'm missing something essential.

I never suggested eliminating the wraparound check, only simplifying the
overflow check.  You could combine the wraparound and overflow into one:

if (sec_start + sec_count < sec_count ||
    sec_start > (INT64_MAX >> BDRV_SECTOR_BITS) - sec_count) {
    return false;
}

Remember, sec_start and sec_count were both typed as unsigned 64-bit
values, so everything in the above computation is well-defined
arithmetic, and you catch all cases of trying to add two numbers into
something that doesn't fit in 64 bits, as well as all cases of the
addition fitting in 64 bits but going beyond the maximum possible sector
number (since it is not possible to have a sector number whose
corresponding offset would exceed 63 bits).

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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